BLACK RIVER BRACHIOPODS 
75 
lamellose. From R. dentatum (Hall), which also possesses a 
single plication in the sinus, R. uniplicatum differs in proportions, 
as well as in smaller size and less gibbous character. Moreover, 
in dentatum the plications of the fold and sinus are quite as angular 
as those of the slopes, while in uniplicatum they are distinctly 
broader and more rounded. As compared with Foerste’s and 
Miller’s figures of R. dentatum arnheimense ( R . arnheimense ) , 
that species is larger, narrower, and characterized by a more 
curved anterior margin than the one here considered. 
Formation and Locality : Kimmswick Limestone, — near Sul- 
phur Springs, Mo., and Batchtown, 111. 
Cotypes, No. 27,450, Walker Museum. 
ZYGOSPIRA VARIABILIS sp. nov. 
Plate II, Figs. 7-9. 
Shell small, elliptical ; wider than long in adults but longer than 
wide in young. Dimensions of three typical specimens : Length, 
5.2 mm., 6.5 mm., and 7.5 mm. ; width, 5.6 mm., 6.9 mm., and 
7.7 mm. ; thickness, 2.9 mm., 3.8 mm., and 4.6 mm. 
Pedicle valve pronouncedly convex in the umbo, moderately so 
on the slopes, and depressed or concave near the cardinal extremi- 
ties. Beak prominent, sharp, incurved, and produced from 0.3 to 
0.9 mm. beyond that of the brachial valve. At the crest of the 
low, rounded mesial fold runs a slight longitudinal depression, 
manifested as a deepened and very broad furrow between two 
of the plications. Brachial valve less convex than the pedicle ; 
greatest convexity in the umbo ; slopes flattened. Near the beak 
originates the broad, shallow, undefined mesial sinus, which may 
or may not be marked by a broadened median plication to corre- 
spond to the median furrow of the fold. Width of sinus in a 
specimen 6.7 mm. broad is approximately 4 mm. Surface of both 
valves marked by eighteen to twenty-two simple, rounded or sub- 
angular plications. 
What relation this species may hold to Z. tantilla Bradley 1 
cannot be determined. Bradley does not figure his species, nor 
does his description afford anything determinative ; it might apply 
to the New York Z. recurvirostris. Therefore, since Z tantilla 
is neither recognizably figured nor defined, it should be abandoned. 
Z. variabilis differs from the typical Z. recurvirostris (Hall and 
l Bull. Mus. Comp. Zool., 64, p. 525. 
